The present invention relates to a hydraulic power steering system having a servomotor configured as a hydrostatic motor assembly or as a hydraulic displacer assembly which also acts as a steering damper. Damper-valve arrangements are provided on hydraulic lines between the servomotor and a servo valve, and the damper-valve arrangements having a throttle resistance controllable as a function of temperature.
Present-day motor vehicles are usually equipped with a power steering system which typically operates with auxiliary hydraulic power, at least where passenger vehicles are concerned. The forces to be exerted by the driver when the vehicle steering system is actuated are always kept sufficiently low.
In order essentially to avoid or suppress vibrations in the steering system, virtually all vehicle steering systems have a steering damper.
DE 28 38 151 A1 discloses a power steering system in which the generation of a boosting force is provided only at a low travelling speed and the servomotor/steering-damper assembly performs merely damping functions at higher travelling speeds. So that changes in viscosity of the hydraulic medium in response to changing temperatures can be compensated in terms of the damping effect, temperature-compensated throttles may be provided, in which a throttle screw cooperates with a stop having a variable temperature coefficient.
DE 40 29 156 A1 and DE 41 06 310 A1 describe how to cause a double-acting piston/cylinder assembly serving as a servomotor to act constantly as a steering damper.
These known damper-valve arrangements provided are, however, comparatively complicated. According to DE 43 23 179 C1, in order to simplify the damper-valve arrangements, the damper valves is formed on a perforated-disc-like valve-carrier part. The holes, through which hydraulic medium passes, can be controlled by valve reeds and/or sprung valve plates which are held on the end face of the valve-carrier part by bolts. In this case, there may be provision for clamping the perforated-disc-like valve-carrier part in the manner of a spacer ring or of a spacer washer between a bearing face of a nipple, arranged on the housing of the servo valve or of the servomotor, and a counterbearing face of the connecting part of the hydraulic line, with the connecting part being connectable to the nipple.
Instead, the valve-carrier part can also be held captive in the nipple or in the connecting part, as illustrated, by way of example, in DE 44 23 658 A1.
DE 36 05 207 A discloses a hydraulic vibration damper which a piston/cylinder assembly and in which a piston working space is connected to a compensating space via a bottom valve. The bottom valve is a damper-valve arrangement having a carrier part in the form of a circular disc, with axial bores and with a central bolt which, on one hand, holds spring plates in the form of a circular disc for controlling some of the axial bores and, on the other hand, on that side of the carrier part facing away from the spring plates, guides an annular disc for controlling the other axial bores. An axial duct is formed in the bolt. The axial duct is controlled as a function of temperature by a bimetallic spring which, in turn, is mounted on bolt-side holding devices.
An object of the present invention, then, is to improve even further a power steering system and, in particular, to adapt the operating behavior to different external influences.
This object is achieved, according to the invention, in that, in a power steering system, the damper-valve arrangements each have at least one duct with a closing or throttle member which is tensioned into its closing or throttling position by a spring arrangement having a highly temperature-dependent spring force markedly decreasing at a falling temperature.
The invention is based on the general recognition of controlling the throttle resistance, and consequently the damper effect of the damper-valve arrangement, as a function of temperature, in order to achieve adaptation to the temperature-dependent viscosity of the hydraulic medium. At very low temperature, the hydraulic medium becomes markedly more viscous, with the result that the hydraulic medium can flow less easily through the damper-valve arrangements. According to the invention, then, this effect is largely compensated in that unthrottling of the damper-valve arrangements takes place at low temperature.
The spring arrangements operating as a function of temperature may have springs consisting of a form memory alloy and/or bimetallic springs. As regards springs consisting of a form memory alloy, the spring behavior changes abruptly when a narrow transition-temperature range is exceeded or undershot. Bimetallic springs may change their behavior comparatively xe2x80x9ccreepinglyxe2x80x9d within a wide temperature range or xe2x80x9csnap overxe2x80x9d at a temperature threshold value. Optimum adaptation to the changes in viscosity of the hydraulic medium can be achieved by an appropriate selection or combination of the spring elements.
The present invention is of great importance particularly in very cold climatic zones. When the vehicle is travelling in a straight line for lengthy periods, there is no inflow or outflow of hydraulic medium on the servomotor. Since the servomotor is exposed to the slipstream and/or splashwater, the hydraulic medium then remains very cold and viscous. As a result, any hydraulic flow can take place only against markedly increased flow resistance and the steering may become sluggish. These adverse effects are avoided with the invention.